This application relates to polycarbonate-polysiloxane copolymers and polycarbonate-polysiloxane/polycarbonate blends having good transparency and good heat resistance and the process for preparing the same.
Polycarbonate is a type of plastic that is used for many applications that require both strength and clarity (e.g., eyeglass lenses, windows, etc.). The most widely produced polycarbonate is a homopolymer made by polymerizing Bisphenol A (“BPA”). Unfortunately, for some applications, such as automotive lighting lenses and films used in optical displays, the glass transition temperature (Tg) (150° C.) of BPA homopolymer is too low to prevent the part from softening or melting under typical use conditions. It is known in the art that the heat resistance of BPA polycarbonate can be improved through incorporation of a high heat co-monomer, such as menthane bisphenol (BHPM), into the BPA polycarbonate polymer chains. Copolymers of BHPM, such as 4,4′-[1-methyl-4-(1-methyl-ethyl)-1,3-cyclohexandiyl]-bisphenol (1,3-bis-hydroxyphenyl menthane, hereinafter referred to as 1,3-BHPM) and 2,8-di-(4-hydroxyphenyl)menthane (referred to as 2,8-BHPM), and BPA are described in U.S. Pat. No. 5,480,959 to GE (Schmidhauser). Unfortunately, while these materials have a high Tg, they suffer from minimal ductile impact (i.e., inferior toughness), even at room temperature. Thus, a transparent polycarbonate-polysiloxane copolymer with a Tg above 150° C. and good impact properties would be a desirable material.
Polycarbonate-polysiloxane copolycarbonates of bisphenol A (BPA) and siloxane comonomers are known to have excellent impact resistance properties in comparison with BPA homopolycarbonates, especially at lower temperatures. Such materials have found commercial use in articles such as helmets and automobile parts, and many other applications requiring impact resistance. Also, BPA/siloxane copolymers have enhanced flame-retardant properties in comparison with BPA polycarbonate, and have been successfully been used to replace halogenated flame retardant products for some applications requiring this performance.
Unfortunately, BPA/siloxane copolymers have proven difficult to manufacture at commercial scale because while BPA homopolycarbonate may be used in applications requiring clarity (e.g., eyeglass lenses and optical disks) it has proven difficult to make clear (i.e., high % transmission and low haze) BPA/siloxane copolymers. Also, the difficulty in making a transparent copolycarbonate adversely affects manufacturing change-over between products because large amounts of “off specification” products are made when changing back and forth between making clear BPA homopolycarbonate and unclear BPA/siloxane copolymers.
A previous attempt to make transparent BPA/siloxane copolymers is described by Phelps and coworkers in U.S. Pat. No. 5,530,083 (“'083”). In '083, Phelps et al. disclose a process (“Phelps method”) which comprises adding phosgene to a bisphenol under interfacial reaction conditions and at a pH1 in the range of from about 10 to about 12 in the presence of an effective amount of phase transfer catalyst. After about 1 to about 99 mole percent of phosgene was added (based on the total moles of available hydroxyl groups of the bisphenol), the pH was lowered to a value in the range of about 8 to about 9 (pH is a logrhythmic scale so the acid concentration decreases by a factor of 10× going from 10 to 9). Phosgene addition was continued, while maintaining the pH range until there was present at least a sufficient amount, and up to 5% mole % excess of phosgene which is adequate to generate enough chloroformate end groups capable of reacting with available biphenol hydroxyl groups and any hydroxyaryl groups present in the terminal position of polydiorganosiloxane weight percent requirements in the resulting siloxane polycarbonate block copolymer. Next, the predetermined weight percent of the hydroxyaryl polydiorganosiloxane was added, and the pH of the resulting mixture was raised to a value of about 10 to about 12. Finally, excess chloroformate groups were removed (e.g., by adding triethyl amine and/or a chainstopper).
The Phelps method produced a more random copolymer since no phosgene and few short BPA oligomers were present to react and form carbonates with neighboring siloxane oligomers. In prior methods, the BPA had been present simultaneously with the phosgene and siloxane, leading to formation of two separate block copolymers due to reactivity differences between the BPA and the siloxane. It was believed that reactions carried out by the Phelps method circumvented the differences in reactivity between the hydroxyaryl polydiorganosiloxane and BPA. These reaction mixtures were characterized by a single homogenous organic phase. The Phelps method produced a more random distribution of the siloxane and resulted in a more transparent product. These materials were further characterized after extrusion either as homopolymers or as blends with BPA homopolycarbonate. Such copolymers prepared with 5 wt % final siloxane concentration resulted in a haze of 6.9, while a siloxane copolymer made by this method and blended with polycarbonate to produce a 5 wt % siloxane composition had a haze of 27.8. While this represented a significant improvement over pre-existing commercial processes which produced haze values of greater than 60 for both copolyers and blends, more work was necessary to make a clear, commercially-viable product.
It would also be desirable to make a low haze, high impact, heat resistant product because for some applications, such as automotive lighting lenses and films used in optical displays, the glass transition temperature (Tg) (150° C.) of BPA homopolymer is inadequate. It is known in the art that the heat resistance of BPA polycarbonate can be improved through incorporation of a high heat monomer, such as menthane bisphenol (BHPM), into the BPA polycarbonate polymer chains. Copolymers of BHPM, such as 4,4′-[1-methyl-4-(1-methyl-ethyl)-1,3-cyclohexandiyl]-bisphenol (1,3-bis-hydroxyphenyl menthane, hereinafter referred to as 1,3-BHPM) and 2,8-di-(4-hydroxyphenyl)menthane (referred to as 2,8-BHPM), and BPA are described in U.S. Pat. No. 5,480,959 to GE (Schmidhauser). Unfortunately, these materials suffer from minimal ductile impact, even at room temperature. A transparent polycarbonate copolymer with a Tg above 150° C. and good impact properties would be ideal.